Field effect transistor including πconjugate polymer and liquid crystal display including the field effect transistor

ABSTRACT

The present invention relates to a field effect transistor (FET element) in which a π-conjugated polymer film serving as a semiconductor layer is manufactured by first forming a π-conjugated polymer precursor film using a π-conjugated precursor which is soluble in a solvent and then changing the precursor polymer film to the π-conjugated polymer film. A liquid crystal display apparatus uses the FET element as an active drive element. A large number of the FET elements can be manufactured on a large area substrate at the same time at lost cost and operate stably. A large current flow between the source and drain can be significantly modulated by a voltage applied to the gate of the FET element.

This disclosure is a continuation of patent application Ser. No.07/965,536, filed Oct. 23, 1992, now abandoned which is a continuationof patent application Ser. No. 07/576,437, filed as PCT/JP90/00017 Jan.10, 1990, now abandoned.

TECHNICAL FIELD

The present invention relates to a field effect transistor (referred toas an FET hereinafter) using an organic semiconductor and a liquidcrystal display apparatus using the same as a drive element.

BACKGROUND ART

Monocrystalline silicon and GaAs are used in practical FETs. However,these materials are expensive, and the process for manufacturing theFETs is very complicated. In addition, the area of an FET is limited bythe size of a silicon or gallium arsenide wafer. For example, when anactive drive element used in a wide screen liquid crystal displayelement is manufactured, there are substantial restrictions on costs andarea when a silicon or gallium arsenide wafer is used. Because of suchrestrictions, when an FET is used as a drive element in a liquid crystaldisplay, a thin film transistor using amorphous silicon is presentlyused. However, in a thin film transistor using amorphous silicon, itbecomes increasingly difficult to manufacture many elements uniformly ona plane surface at low cost as the display element area is increased.Thus, recently, it has been proposed that FETs be manufactured using anorganic semiconductor material. Among organic semiconductor materials,those using a π-conjugated polymer are especially useful because theyare easily processed, which is a characteristic of a polymer material,and area can be easily increased (see Japanese Patent Publication62-85224).

It is thought that a π-conjugated polymer, whose chemical structureincludes a conjugate double bond or triple bond, has a band structurecomprising a valence band, a conduction band, and a forbidden bandseparating the valence and conduction bands which is formed byoverlapping of π-electron orbits. The forbidden band of the π-conjugatedpolymer is mostly within a range of 1 to 4 Ev which varies with thematerial. Therefore, the π-conjugated polymer itself has the electricalconductivity of an insulator or close to it. However, charge carriersare generated by removing electrons from the valence band (oxidation)chemically, electrochemically, physically, or the like or by implanting(referred to as doping hereinafter) electrons into the conduction band(reduction). As a result, it is possible to vary electrical conductivityover a large range, from insulating to metallic conduction, bycontrolling the amount of doping. The π-conjugated polymer obtained byan oxidation reaction is p-type and by a reduction reaction is n-type.This result is similar to addition of dopant impurities to an inorganicsemiconductor. Thus, it is possible to manufacture various semiconductorelements using the π-conjugated polymer as a semiconductor material.

It is known that polyacetylene can be used as a π-conjugated polymer inan FET (Journal of Applied Physics, Volume 54, page 3255, 1983). FIG. 15is a sectional view showing a conventional FET using polyacetylene. Inthis figure, reference numeral 1 designates a glass substrate, referencenumeral 2 designates an aluminum gate electrode, reference numeral 3designates a polysiloxane insulating film, reference numeral 4designates a polyacetylene semiconductor layer, and reference numerals 5and 6 designate gold source and drain electrodes, respectively.

In operation of the polyacetylene FET, when a voltage is applied betweenthe source electrode 5 and the drain electrode 6, a current flowsthrough the polyacetylene film 4. When a voltage is applied to the gateelectrode 2 disposed on the glass substrate 1 and separated from thepolyacetylene film 4 by the insulating film 3, the electricalconductivity of the polyacetylene film 4 can be varied by the resultingelectric field so that the current flow between the source and drain canbe controlled. It is thought that the width of a depletion layer in thepolyacetylene film 4 adjacent to the insulating film 3 varies with thevoltage applied to the gate electrode 2 and thereby the area of acurrent channel varies. However, the amount of the current flow betweenthe source and drain that can be varied by the gate voltage is small inthis FET.

In other FETs, the π-conjugated polymer is poly (N-methylpyrrole)(Chemistry Letters, page 863, 1986) and polythiophene (Applied PhysicsLetters, Volume 49, page 1210, 1986). FIG. 16 is a sectional view of anFET in which poly (N-methylpyrrole) or polythiophene is used as asemiconductor layer. In this figure, reference numeral 3 is a siliconoxide insulating film, reference numeral 4 is a poly (N-methylpyrrole)or polythiophene semiconductor layer, reference numerals 5 and 6 aregold source and drain electrodes, respectively, reference numeral 1 is asilicon substrate and gate electrode, and reference numeral 2 is a metalmaking ohmic contact to the silicon substrate 1. When poly(N-methylpyrrole) is used as the semiconductor layer, a current flowingbetween the source electrode 5 and the drain electrode 6 through thesemiconductor layer 4 is only slightly controlled by the gate voltage,so there is no practical value in this FET.

On the other hand, when polythiophene is used as the semiconductorlayer, the current flowing between the source electrode 5 and the drainelectrode 6 through the semiconductor layer 4 can be modulated 100 to1000-fold by the gate voltage. However, since polythiophene is formed byelectrochemical polymerization in the prior art, it is very difficult tomanufacture many uniform FETs at the same time.

Thus, the current between the source and drain which can be modulated bythe gate voltage is too small in an FET in which polyacetylene or poly(N-methylpyrrole) is used as the semiconductor layer. In addition,although the current between the source and drain can be highlymodulated by the gate voltage in an FET using polythiophene as thesemiconductor layer and the FET is highly stable, since the FET ismanufactured by forming a polythiophene film directly on the substrateby electrochemical polymerization, it is difficult to manufacture manyuniform FETs on a large substrate at the same time, which is a problem.

DISCLOSURE OF INVENTION

In an FET in accordance with the present invention, a π-conjugatedpolymer film semiconductor layer is formed by first forming aπ-conjugated polymer precursor film using a π-conjugated polymerprecursor which is soluble in a solvent and then changing the precursorpolymer film to the π-conjugated polymer film. In a liquid crystaldisplay apparatus in accordance with the present invention, the FET isused as an active drive element therein.

In the present invention, instead of directly forming the π-conjugatedpolymer film by a method such as electrochemical polymerization, theπ-conjugated polymer precursor film is formed from a π-conjugatedpolymer precursor which is soluble in a solvent, and then this precursorpolymer film is changed to the π-conjugated polymer film. By using thisπ-conjugated polymer film as the semiconductor layer, the process formanufacturing the element is significantly simpler. As a result, it ispossible to manufacture many FETs on a large area substrate at the sametime and at low cost, to stably operate all of the manufactured FETs,and to highly modulate a current flowing between the source and drainwith a gate voltage. In addition, when the FET is used as a driveelement in a liquid crystal display apparatus, the area can easily beincreased and a highly functional liquid crystal display apparatus canbe manufactured at low cost.

In an FET in accordance with another aspect of the invention, theπ-conjugated polymer film semiconductor layer is formed by first forminga Langmuir-Blodgett (referred to as LB hereinafter) film of aπ-conjugated polymer precursor using a π-conjugated polymer precursorwhich is soluble in a solvent, and then changing this LB film of theprecursor polymer to an LB film (although this LB film is an organicthin film, it is referred to as the LB film in a broad sense) of theπ-conjugated polymer. In addition, in a liquid crystal display apparatusin accordance with the invention, the FET is used as an active driveelement.

According to still another aspect of the invention, laminated films,comprising a semiconductor layer of the π-conjugated polymer obtainedfrom a precursor which is soluble in a solvent and an acid giving filmwhich produces an acid in a reaction in which the above π-conjugatedpolymer is obtained from a precursor which is soluble in a solvent, aresandwiched by the source electrode and the drain electrode. As a result,the π-conjugated polymer precursor film can be effectively changed tothe π-conjugated polymer film, and a current flow between the source anddrain can be highly modulated by a gate voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is sectional view showing an FET in accordance with an embodimentof the present invention;

FIG. 2 is a sectional view of one pixel of a liquid crystal displayapparatus in accordance with an embodiment of the present invention;

FIG. 3 and 4 are sectional views, each showing an FET, accordance withanother embodiment of the present invention;

FIG. 5 is a sectional view of one pixel of a liquid crystal displayapparatus in accordance with another embodiment of the presentinvention;

FIG. 6 is a graph showing current flow between a source and a drain as afunction of the voltage between the source and drain, with gate voltageas a parameter, of an FET according to a first embodiment of theinvention;

FIGS. 7, 8, an 9 are graphs showing the same characteristics as FIG. 6for second, third, and fourth embodiments, respectively;

FIG. 10 is a graph showing current flow between the source drain as afunction of gate voltage of an FET according to first and fourthembodiments of the invention and a comparative FET where the voltagebetween the source and drain is -50 V;

FIG. 11 is a graph showing characteristics of second and third FETembodiments according to the invention and of a comparative FET;

FIG. 12 is a graph showing current flow between the source and drain andas a function of the voltage between the source and drain, with gatevoltage as a parameter, of an FET in a liquid crystal display apparatusaccording a fifth embodiment of the invention;

FIGS. 13 and 14 are graphs showing the same characteristics as FIGS. 12for sixth and seventh embodiments of the invention;

FIG. 15 is a sectional view showing a conventional FET usingpolyacetylene as a semiconductor layer; and

FIG. 16 is a sectional view showing a conventional FET using poly(N-methylpyrrole) or polythiophene as a semiconductor layer.

The same reference numerals are used in all the figures to designate thesame or corresponding parts.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a view of an example of an FET in accordance with the presentinvention. In FIG. 1, reference numeral 1 designates a substrate,reference numeral 2 designates a gate electrode disposed on thesubstrate 1, reference numeral 3 designates an insulating film,reference numeral 4 designates a π-conjugated polymer film or its LBfilm as a semiconductor layer, and reference numerals 5 and 6 designatesource and drain electrodes, respectively.

FIG. 2 is a sectional view showing an example of a liquid crystaldisplay apparatus in accordance with the present invention. In FIG. 2,reference numeral 1 designates a substrate, reference numeral 2designates a gate electrode disposed on one side of the substrate 1,reference numeral 3 designates an insulating film disposed on thesubstrate 1 and the gate electrode 2, reference numeral 5 designates asource electrode disposed on the insulating film 3, reference numeral 6designates a drain electrode also disposed on the insulating film 3 andisolated from the source electrode 5, and reference numeral 4 designatesa semiconductor layer comprising a π-conjugated polymer or its LB filmand disposed on the insulating film 3, the source electrode 5, and thedrain electrode 6, in contact with the source and drain electrodes 5 and6, respectively, in which the elements 2 to 6 comprise part of the FET11 in the liquid crystal display apparatus. In addition, referencenumeral 7 designates an electrode connected to the drain electrode 6 ofthe FET 11, reference numeral 8 designates a liquid crystal layer,reference numeral 9 designates a transparent electrode, and referencenumeral 10 designates a glass polarizing plate for processing.Orientation is performed on the electrodes 7 and 9. The elements 7 to 10comprise a part 12 of the liquid crystal display in the liquid crystaldisplay apparatus.

The materials used in FETs and liquid crystal display apparatus inaccordance with the present invention are as follows.

The substrate 1 can be formed of any insulating material. Morespecifically, it can be glass, an alumina sintered body, or aninsulating plastic, such as polyamide, polyester, polyethylene,polyphenylene sulfide, or polyparaxylene. In addition, the substrate 1is preferably transparent when it is used in the liquid crystal displayapparatus.

The gate electrode 2, the source electrode 5, and the drain electrode 6are a metal, such as gold, platinum, chrome, palladium, aluminum,indium, or molybdenum, low-resistance polysilicon, low-resistanceamorphous silicon, tin oxide, indium oxide, indium tin oxide (ITO), orthe like. However, the materials are not limited to the materials listedabove, and two or more of the above materials can be used together. Inaddition, the electrodes may be made by vacuum deposition, sputtering,plating, or any of several CVD growth methods. In addition, a conductiveorganic group low molecular compound or π-conjugated polymer can beused. In this case, an LB method can also be used.

In addition, p-type silicon or n-type silicon can be used as the gateelectrode 2 and the substrate 1 in the liquid crystal display apparatusin which the FET shown in FIG. 1 or the FET shown in FIG. 2 can be thedriving part. In this case, the substrate 1 can be dispensed with. Inaddition, in this case, although the specific resistivity of the p-typesilicon or n-type silicon can be any value, it is preferably less thanthat of the π-conjugated polymer film 4 serving as the semiconductorlayer. In addition, a conductive plate or film, such as a stainlesssteel plate or a copper plate, can be used as the gate electrode 2 andthe substrate 1 in accordance with the use of the FET.

The insulating film 3 can be formed of any organic or inorganicinsulating material. In general, silicon oxide (SiO₂), silicon nitride,aluminum oxide, polyethylene, polyester, polyamide, polyphenylenesulfide, polyparaxylene, polyacrylonitrile, insulating LB films, orseveral of these can be used together. There is no limitation on themethod for forming the insulating film. For example, CVD, plasma CVD,plasma polymerization, deposition, spin coating, dipping, cluster ionbeam deposition, or an LB method can be used. In addition, when p-typesilicon or n-type silicon are used as the gate electrode 2 and thesubstrate 1, a silicon oxide film produced by thermal oxidation of thesilicon or the like is preferably used as the insulating film 3.

The electrode 7 of the FET, which is short-circuited to the drain 6 inthe liquid crystal display 12 of the liquid crystal display apparatus,can be formed of any material which has sufficient electricalconductivity and is insoluble in the liquid crystal material. Forexample, a metal, such as gold, platinum, chrome, or aluminum, atransparent electrode, such as tin oxide, indium oxide, indium tin oxide(ITO), or an electrically conductive organic polymer can be used. Ofcourse, two or more of the above materials can be used together. As amaterial for the electrode 9 on the glass plate 10, a transparentelectrode, such as tin oxide or indium tin oxide (ITO), is used ingeneral. Alternatively, a conductive organic polymer having appropriatetransparency can be used. In addition, two or more of the abovematerials can be used together. However, a process for orientation, suchas diagonal deposition of SiO₂ or rubbing, has to be performed on theelectrodes 7 and 9. A liquid crystal material, such as a guest and hosttype liquid crystal, a TN type liquid crystal, or a smectic C phaseliquid crystal, may be used for the liquid crystal layer 8. When glassis used as the substrate 1 and a transparent electrode is used as theelectrode 7, the contrast ratio is increased by attaching a polarizingplate to the substrate 1. The polarizing plate can be formed of anypolarizing material.

In addition, as a material for the π-conjugated polymer film or the LBfilm 4 as the semiconductor layer, a material having a precursor of aπ-conjugated polymer soluble in a solvent can be used. Of course, two ormore of the materials can be used together. In addition, when the LBfilm of the precursor is formed, a material having an amphipathicproperty is preferably used. Among the materials having a precursor ofthe π-conjugated polymer soluble in the solvent are especially theπ-conjugated polymers represented by the following general formula (1),##STR1## (where R₁ and R₂ are one of H, an alkyl group, and an alkoxylgroup, and n is an integer, at least 10). These materials are excellentwith regard to the characteristics of the FET. In addition, since theprecursor of the π-conjugated polymer can be easily synthesized, aπ-conjugated polymer in which R₁ and R₂ are --H is preferably used. Thesolvents can be organic solvents of several kinds, water, or a mixturethereof.

When the precursor LB film is formed, an organic solvent whose specificgravity is less than that of water and which is not likely to dissolvein water and is likely to evaporate is preferably used. A precursor ofthe π-conjugated polymer in which both R₁ and R₂ are --H in the abovegeneral formula (1), a material represented by the following generalformula (2), ##STR2## (where R₃ is a hydrocarbon group having 1 to 10carbon atoms) is preferably used because of preservation stability. Asthe R₃ in the general formula (2), any hydrocarbon group having 1 to 10carbon atoms can be used. For example, methyl, ethyl, propyl, isopropyl,n-butyl, 2-ethylhexyl, or cyclohexyl groups can be used. Among theabove, a hydrocarbon group having 1 to 6 carbon atoms, especially amethyl or ethyl group, is preferably used. Although there is not anyparticular limitation on the method for synthesizing the polymerprecursor, the polymer precursor obtained by a sulfonium saltdecomposition method which will be described later is preferable becauseof its stability.

As a monomer, when the general formula (2) is obtained by the sulfoniumsalt decomposition method, 2,5-thienylene dialkylsulfonium saltrepresented by the following general formula (3) ##STR3## (where R₄ andR₅ are hydrocarbon groups having 1 to 10 carbon atoms and A⁻ is acounter ion) is used. As the R₄ and R₅ in the general formula (3), anyhydrocarbon group having 1 to 10 carbon atoms can be used. For example,methyl, ethyl, propyl, isopropyl, n-butyl, 2-ethylhexyl, cyclohexyl, orbenzyl groups can be used. Among the above, a hydrocarbon group having 1to 6 carbon atoms, especially methyl or ethyl groups, are preferablyused. There is no particular limitation in the counter ion A⁻, forexample a halogen, a hydroxyl group, boron tetrafluoride, perchloricacid, carboxylic acid, and sulfonic acid ion can be used. Among theabove, halogens, such as chlorine or bromine, or hydroxyl group ions arepreferably used.

When condensation polymerization of the general formula (3) is performedto produce the general formula (2), water, alcohol alone, or a mixedsolvent containing water and/or alcohol is used as a solvent. Analkaline solution is preferably used as a reaction solution incondensation polymerization. The alkaline solution is preferably astrong basic solution of a PH 11 or higher. As an alkali, sodiumhydroxide, potassium hydroxide, calcium hydroxide, quaternary ammoniumsalt hydroxide, sulfonium salt hydroxide, a strong basic ion exchangeresin (OH type), or the like is used. Among them, especially, sodiumhydroxide, potassium hydroxide or quaternary ammonium salt hydroxide, orstrong basic ion exchange resin are preferably used.

Since sulfonium salt is unstable in response to heat and light,especially ultraviolet rays, and is strongly basic,desulfonium-saltation gradually occurs after condensation polymerizationwith the result that the salt is not effectively changed to an alkoxylgroup. Therefore, it is desirable that a condensation polymerizationreaction occur at relatively low temperature, more specifically, 25° C.or less, or further -10° C. or less. The reaction time can be determinedin accordance with polymerization temperature and is not particularlylimited. However, it is usually within a range of 10 minutes to 50hours.

According to the sulfonium salt decomposition method, afterpolymerization, the precursor of the π-conjugated polymer is generatedas a polymer electrolyte (polymer sulfonium salt) having a highmolecular weight and including sulfonium salt, that is ##STR4## (A⁻) atthe side chain and the sulfonium salt side chain reacts with alcohol (R₃OH) in the solution and the alkoxyl group (corresponding to OR₃ in theformula (2)) of the alcohol becomes the side chain. Therefore, thesolvent to be used has to contain an alcohol above R₃ OH. The alcoholcan be used alone or with another solvent. Although any solvent which issoluble in alcohol can be used with alcohol, water is preferably used inpractice. Although the ratio of alcohol in the mixed solvent can be anyvalue, alcohol in 5 or more weight percent is preferably used.

The sulfonium side chain can be effectively substituted for the alkoxylgroup if a reaction in which the sulfonium side chain is substituted forthe alkoxyl group is performed at a temperature higher than thecondensation polymerization temperature in a solvent containing alcoholafter the condensation polymerization. When the polymerization solventcontains alcohol, substitution for the alkoxyl group can be performedafter the polymerization. On the other hand, when the polymerizationsolvent does not contain alcohol, for example, when it is water, thealcohol is mixed into it after the polymerization and then the samereaction can be performed. The substitution for the alkoxyl group ispreferably performed at a temperature of 0° C. to 50° C. and morepreferably 0° C. to 25° C. considering reaction speed. Since a polymerhaving an alkoxyl group at its side chain is insoluble in the mixedsolvent in general, it is precipitated as the reaction proceeds.Therefore, the reaction is preferably continued until the polymer isfully precipitated. Thus, the reaction time is preferably 15 minutes ormore and, more preferably, 1 hour or more to improve yield. Theprecursor of the π-conjugated polymer having an alkoxyl group at itsside chain is isolated by filtering the precipitate.

In order to obtain a useful precursor of the π-conjugated polymer, withhigh enough molecular weight and a repeat unit n of at least 10, morepreferably 20 to 50,000, of the π-conjugated polymer precursor of thegeneral formula (2), for example, having a molecular weight of 3500 ormore which is not dialyzed by dialysis processing, a dialysis film iseffectively used.

The precursor of the π-conjugated polymer shown in the general formula(2) having a group such as an alkoxyl group in its side chain has highsolubility and is soluble in many kinds of organic solvents. Theseorganic solvents are, for example, dimethylformamide, dimethylactamide,dimethylsulfoxide, dioxane, chloroform, and tetrahydrofuran.

When the LB film of the precursor is formed, an organic solvent having aspecific gravity less than that of water and not likely to dissolve inwater and likely to evaporate is preferably used.

As a method for producing the precursor of the π-conjugated polymer usedin the present invention, spin coating, casting, dipping, bar coating,or roll coating utilizing the precursor of the π-conjugated polymerdissolved in the solvent is used. Thereafter, the solvent evaporates andthen a π-conjugated polymer precursor thin film is heated, whereby theπ-conjugated polymer semiconductor film is obtained. Although there isnot any particular heating condition, when the π-conjugated polymerprecursor thin film is heated to become the π-conjugated polymer film,it is preferably heated to 200° C. to 300° C. in an inert gasatmosphere. Of course, the π-conjugated polymer precursor thin film canbe changed to the π-conjugated polymer film even if it is heated to 200°C. or less. In addition, when it is heated in an inert gas atmospherecontaining an acid such as HCl or HBr, the π-conjugated polymerprecursor thin film can be smoothly changed to the π-conjugated polymerfilm in most cases.

On the other hand, as a method for obtaining an LB film of theπ-conjugated polymer precursor used in the present invention, verticaldipping using a Kuhn trough, horizontal attaching, and LB film formingusing a moving wall trough, using as a developer a π-conjugated polymerprecursor solution dissolved in a solvent with pure water, or an aqueoussolution with a salt or the like as a subphase, may be used so that theLB film is deposited on a substrate. Thereafter, water evaporates andthen the dried LB film of the π-conjugated polymer precursor isobtained. Then, the LB film of the π-conjugated polymer precursor isheated, whereby the LB film of the π-conjugated polymer serving as asemiconductor is obtained. Although there is not any particular heatingcondition for heating the LB film of the π-conjugated polymer precursorto become the π-conjugated polymer LB film, it is preferably heated to200° C. to 300° C. in an inert gas atmosphere. Of course, the LB film ofthe π-conjugated polymer precursor can be changed to the LB film of theπ-conjugated polymer even if it is heated to 200° C. or less. Inaddition, when it is heated in an inert gas atmosphere containing anacid, such as HCl or HBr, the LB film of the π-conjugated polymerprecursor can be smoothly changed to an LB film of the π-conjugatedpolymer in most cases.

When the LB film of the precursor is formed, even if the precursor ofthe π-conjugated polymer is soluble in a solvent, if it is notamphipathic enough, a spreading solution mixed with a compound having agood amphipathic property, such as stearic acid or arachidic acid, isused for forming the LB film. Alternatively, the LB film can be formedwith the precursor of the π-conjugated polymer adsorbed on amonomolecular film of a compound having an amphipathic property in thesubphase.

As described above, according to the method for forming the π-conjugatedpolymer film, a π-conjugated polymer film or an LB film is not directlyformed like a conventional electrochemical polymerization. First, thepolymer precursor film or its LB film is formed using a π-conjugatedpolymer precursor which is soluble in a solvent, and then it is changedto the π-conjugated polymer film or an LB film. As a result, theπ-conjugated polymer film or LB film can be uniformly and easily formedon a substrate over a large area.

The π-conjugated polymer has a low electrical conductivity but hassemiconductor properties even if undoped. However, doping is oftenperformed to improve characteristics of the FET. There are chemical andphysical methods of doping (referring to "Industrial Material", Volume34, Number 4, page 55, 1986). The former comprises (1) doping from avapor phase, (2) doping from a liquid phase, (3) electrochemical doping,(4) light induced doping, and the like. The latter method comprises ionimplantation. Any of these methods can be used.

FIGS. 3 and 4 are sectional views each showing an FET in accordance withanother embodiment of the present invention. Reference number 13designates an acid giving film for promoting the reaction from theprecursor film of the π-conjugated polymer 4 to the π-conjugated polymerfilm which is disposed on the π-conjugated polymer film 4 in FIG. 3 butis disposed on the substrate 1 and the gate electrode 2 in FIG. 4. InFIG. 3, even if the positions of the π-conjugated polymer film 4 and theacid giving film 13 are exchanged, more specifically, even if the acidgiving film 13 is formed on an insulating film 3, the source electrode5, and the drain electrode 6 and then the π-conjugated polymer film isformed on the acid giving film 13, the completed FET can control thecurrent flow between the source and drain in response to a gate voltage.

FIG. 5 is a sectional view showing a liquid crystal display apparatus inaccordance with another embodiment of the present invention in whichreference numeral 13 designates an acid giving film for promoting thereaction from a precursor film of a π-conjugated polymer to aπ-conjugated polymer which is disposed on the π-conjugated polymer film4.

Other parts in FIGS. 3, 4, and 5 designate the same parts as in FIGS. 1and 2, and the methods for manufacturing them are also the same asalready described.

The acid giving layer 13 is a film which produces an acid to promote thereaction from the precursor of the π-conjugated polymer to theπ-conjugated polymer 4 and does not have any particular limitation. Theacid giving film itself is preferably an insulator. For example, thefollowing films are used: an acid impregnated polymer incorporatingpolyimide, a polyester, a polyethylene, a polyphenylene sulfide, apolyparaxylene, or the like, the above polymers containing an acidgenerating agent such as a Lewis acid amine complex, tertiary amineclass, a Lewis acid diazonium salt, a Lewis acid diallyliodonium salt,or a Lewis acid sulfonium salt, or a film which easily eliminates acidby a reaction of p-xylylene-bis (sulfonium halogenide), its derivative,or the like. There is no particular limitation on the method ofproducing the acid giving film. For example, CVD, plasma CVD, plasmapolymerization, deposition, cluster ion beam deposition, organicmolecular beam epitaxial growth, spin coating, dipping, or an LB methodcan be all used.

As an example, a description will be given of a case where theπ-conjugated polymer represented by the general formula (1) is used asthe semiconductor layer and a π-conjugated polymer represented by thefollowing general formula (4) ##STR5## (where R₆ is one of --H, alkylgroup, alkoxyl group and n is an integer, at least 10) is used as theacid giving film. The general formula (4) has a π-conjugated polymerprecursor represented by the following general formula (5) ##STR6##(where R₆ is one of --H, alkyl group, alkoxyl group, R₇ and R₈ arehydrocarbon groups having 1 to 10 carbon atoms, X⁻ is a halogen, such asBr or Cl, and n is an integer, at least 10). The general formula (5) issoluble in water, and a film can be easily formed by spin coating,casting, dipping, bar coating, roll coating, or the like. Therefore, asa method for forming laminated films comprising the π-conjugated polymerprecursor thin film (the general formula (2)) as the semiconductor layerand the π-conjugated polymer precursor film (the general formula (5)) asthe acid giving film, although there is no particular limitation, it ispreferable that the laminated films are formed by producing theπ-conjugated polymer precursor film (the general formula (2)) as thesemiconductor film by spin coating, casting, dipping, bar coating, rollcoating, or the like using a solution of the π-conjugated polymerprecursor dissolved in a solvent, evaporating the solvent, and thenforming the acid giving film (the general formula (5)) by the samemethod as above. Alternatively, the laminated films may be formed byforming the acid giving film (the general formula (5)) as describedabove and then forming the π-conjugated polymer precursor film (thegeneral formula (2)) as the semiconductor film by spin coating, casting,dipping, bar coating, roll coating, or the like using a solution of theπ-conjugated polymer precursor dissolved in a solvent. Of course, thelaminated layers can be repetitively formed. Thereafter, the thus formedlaminated films are heated, whereby laminated layers of the π-conjugatedpolymer film (the general formula (1)) as the semiconductor film and theinsulating film (the general formula (4)) are formed. There is norestriction on heating when the laminated films comprising theπ-conjugated polymer film (the general formula (1)) and the insulatingfilm (the general formula (4)) are formed by heating the laminated filmscomprising the π-conjugated polymer precursor thin film (the generalformula (2)) and the acid giving film (the general formula (5)).However, they are preferably heated to a temperature of 100° C. to 300°C. in an inert gas atmosphere.

A description will be given of a method when the π-conjugated precursorfilm (the general formula (5)) is used as the acid giving film. Theπ-conjugated polymer precursor film (the general formula (5)) as theacid giving layer is changed to the π-conjugated polymer (the generalformula (4)) by heating. At this time, sulfonium ##STR7## and acid (HX)are left. The remaining acid is diffused into the π-conjugated polymerprecursor film (the general formula (2)) as the semiconductor layer,supplying acid.

In addition, if the acid giving film is an insulator, the acid givingfilm can also serve as the gate insulating film in the FET (FIG. 4). Inthis case, the process for manufacturing the FET is simplified.

Next, a description will be given of an operating mechanism of the thusformed FET and a liquid crystal display apparatus in which the FET is adrive element by describing a liquid crystal display apparatus.

Although the operating mechanism is still unknown in many respects, itis believed that the width of a depletion layer formed at a surface ofthe π-conjugated polymer film 4 or its LB film is controlled by avoltage applied between the gate electrode 2 and the source electrode 5at the interface between the π-conjugated polymer film or its LB film 4and the insulating film 3. As a result, an effective channelcross-sectional area varies so that the current flowing between thesource electrode 5 and the drain electrode 6 varies. When theπ-conjugated polymer film 4 or its LB film has the properties of ap-type semiconductor having low electrical conductivity, even if p-typesilicon, n-type silicon, or an organic electrically conductive polymerwhich has high electrical conductivity is used as the gate electrode 2instead of a metal electrode, it is believed that a depletion layerhaving a large width is formed in the π-conjugated polymer film 4 or itsLB film, whereby an electrical field effect occurs.

In a liquid crystal display apparatus in accordance with the presentinvention, the FET 11 is connected in series to the liquid crystaldisplay 12. When the π-conjugated polymer film or its LB film has theproperties of a p-type semiconductor and a negative voltage is appliedto the gate electrode 2 while a negative voltage is applied to thetransparent electrode 9 using the source electrode 5 as a reference, theliquid crystal material 8 is turned on. It is believed that theresistance between the source and drain electrodes of the FET isdecreased by applying the negative voltage to the gate electrode 2 whilea voltage is applied to the liquid crystal display 12. When no gatevoltage is applied and a negative voltage is applied to the transparentelectrode 9 using the source electrode 5 as a reference, the liquidcrystal material 8 is not turned on. It is believed that the resistancebetween the source and drain electrodes of the FET is increased so thatno voltage is applied to the liquid crystal display 12. As describedabove, in a liquid crystal display apparatus in accordance with thepresent invention, operation of the liquid crystal display 12 can becontrolled by varying the gate voltage applied to the attached FET.

Although the gate electrode 2 is disposed on the substrate 1 in FIG. 2,the π-conjugated polymer film or its LB film are disposed on thesubstrate, the source electrode and the drain electrode are separatelyformed thereon, and the insulating film is interposed between the sourceand drain electrodes, and, the gate electrode is formed on theinsulating film. Alternatively, the gate electrode may be formed on thesubstrate, the insulating film interposed between them, the π-conjugatedpolymer film or the LB formed thereon, and then the source electrode andthe drain electrode are separately formed thereon. In anotheralternative, the source electrode and the drain electrode are separatelyformed on the substrate, the π-conjugated polymer film of the LB film isformed thereon, the insulating film is interposed between them, and thenthe gate electrode is formed.

Although the acid giving film 13 is formed on the π-conjugated polymerfilm 4 as the semiconductor layer in FIG. 3, the gate electrode 2 may bedisposed on the substrate 1, the insulating film 3 interposed betweenthem, the source electrode 5 and the drain electrode 6 formed thereon,the acid giving film 13 formed thereon, and the π-conjugated polymerfilm 4 as the semiconductor layer formed thereon. Alternatively, asshown in FIG. 4, the gate electrode 2 may be disposed on the substrate1, the acid giving layer 13 formed thereon, the source electrode 5 andthe drain electrode 6 formed thereon, and then the π-conjugated polymerfilm 4 as the semiconductor layer is formed thereon in which the acidgiving film 13 is also the gate insulating film 3. In still anotheralternative, the gate electrode 2 is disposed on the substrate 1, theacid giving layer 13 as the insulating film 3 is formed thereon, theπ-conjugated polymer film 4 as the semiconductor layer is formedthereon, and then the source electrode 5 and the drain electrode 6 areformed thereon.

In yet a further alternative, the gate electrode 2 is disposed on thesubstrate 1, the insulating film 3 is interposed between them, theπ-conjugated polymer film 4 is formed thereon, the acid giving film 13is formed thereon, and then the source electrode 5 and the drainelectrode 6 are formed thereon. The source electrode 5 and the drainelectrode 6 may be disposed on the substrate 1, the π-conjugated polymerfilm 4 formed thereon, the insulating film 3 as the acid giving film 13is interposed between them, and then the gate electrode 2 is formedthereon.

Although the FET 11 and the liquid crystal display 12 are disposed onthe same substrate in FIGS. 2 and 5, they can be formed on differentsubstrates and then connected.

Although specific embodiments of the present invention will be describedhereinafter, the present invention is not limited to them.

Embodiment 1

A 3-inch n-type silicon substrate having a resistivity of 4 to 8 Ωcm washeated in an oxygen flow and then covered with a silicon oxide filmhaving a thickness of 3000 Å. Then five pairs of gold electrodes with athickness of 300 Å each having an under layer of chrome with a thicknessof 200 Å were deposited on one side of the silicon oxide film in aconventional vacuum deposition process, photolithography process, oretching. The five pairs of gold electrodes served as source and drainelectrodes in an FET. The width of the pair of gold electrodes, that is,the channel width, was 2 mm and the distance between them, that is, thechannel length, was 16 microns. The thus formed substrate will bereferred to as an FET substrate hereinafter.

The temperature of the FET substrate and of its ambient wereapproximately 60° C. and a precursor film was formed on the FETsubstrate by spin coating using a dimethylformamide (DMF) solution withapproximately 2 wt % of a poly (2,5-thienylene vinylene) precursorhaving the following chemical structure: ##STR8## The rotational speedof the spinner was 2000/min. The thickness of the precursor film wasapproximately 800 Å.

Then, the FET substrate covered with a poly (2,5-thienylene vinylene)precursor film was heated to 270° C. for two hours in a nitrogen flow inan infrared image furnace. As a result, the color of the precursor filmchanged from light yellow to brown. The poly (2,5-thienylene vinylene)precursor film changed to the poly (2,5-thienylene vinylene) filmthrough the heating treatment and accordingly absorption in accordancewith ##STR9## at 1590 cm⁻¹ occurred in an infrared absorption spectrum.

Then, the silicon oxide film on the other surface of the FET substratewas removed and an alloy of gallium and indium was attached to theuncovered silicon surface, whereby ohmic contact was made.

The silicon substrate itself served as a common gate electrode of thefive FETs, and the silicon oxide film on the silicon substrate served asa common gate insulating film of the five FETs. Thus, the FET shown inFIG. 1 was produced. In addition, reference numerals 1 and 2 designatethe silicon substrate and the gate electrode, reference numeral 3designates the silicon oxide insulating film, reference numeral 4designates the poly (2,5-thienylene vinylene) film obtained from thepoly (2,5-thienylene vinylene) precursor film as the semiconductor film,and reference numerals 5 and 6 designate gold source and drainelectrodes, respectively.

Embodiment 2

The FET substrate made in accordance with embodiment 1 was used. Atemperature of a subphase (water) was 20° C., and an LB film of aprecursor was formed on the FET substrate by dipping using a Kuhn troughcontaining 0.5 ml of a dimethylformamide (DMF) solution withapproximately 2 wt % of a poly (2,5-thienylene vinylene) precursorhaving the following chemical structure ##STR10## and 9.5 ml ofchloroform as a spreading solution. At this time, the surface tension τwas set at 20 mN/m. The number of layers of the precursor LB film was100.

Then, the FET substrate covered with the LB film of the poly(2,5-thienylene vinylene) precursor was heated at 210° C. for two hoursin a nitrogen flow in an infrared image furnace. As a result, the colorof the precursor LB film changed from light yellow to brown. The LB filmof the poly (2,5-thienylene vinylene) precursor changed to an LB film ofpoly (2,5-thienylene vinylene) through the above heating treatment andaccordingly absorption in accordance with ##STR11## at 1590 cm⁻¹occurred in the infrared absorption spectrum.

The silicon oxide film on the other surface of the FET substrate wasremoved and an alloy of gallium and indium was applied to the uncoveredsilicon surface, whereby ohmic contact was made.

The silicon substrate itself served as a common gate electrode of thefive FETs and the silicon oxide film on the silicon substrate served asa common gate insulating film of the five FETs. Thus, the FET shown inFIG. 1 was produced. In addition, reference numerals 1 and 2 designatethe silicon substrate and also the gate electrode, reference numeral 3designates the silicon oxide insulating film, reference numeral 4designates the LB film of poly (2,5-thienylene vinylene) obtained fromthe LB film of the poly (2,5-thienylene vinylene) precursor serving asthe semiconductor film, and reference numerals 5 and 6 designate goldsource and drain electrodes, respectively.

Embodiment 3

Another example using a heat treatment different from that of embodiment2 to provide the structure of the FET shown in FIG. 1 is describedhereinafter.

Similar to embodiment 2, an LB film (100 layers) of poly (2,5-thienylenevinylene) was formed on the FET substrate by the LB method. However, inthis embodiment, a platinum electrode 300 Å thick having an under layerformed of chrome 200 Å thick was used instead of the gold electrode onthe FET substrate.

Then, the FET substrate covered with the LB film of a poly(2,5-thienylene vinylene) precursor was heated to 90° C. for one hourand a half in a nitrogen flow containing hydrogen chloride gas in aninfrared image furnace. As a result, the color of the precursor LB filmchanged from light yellow to dark purple, inclining toward a metallicluster. The LB film of the poly (2,5-thienylene vinylene) precursorcompletely changed to an LB film of poly (2,5-thienylene vinylene)through the above heating treatment and accordingly absorption inaccordance with ##STR12## at 1590 cm⁻¹ appeared, in and absorption inaccordance with ##STR13## at 1099 cm⁻¹ disappeared from, the infraredabsorption spectrum.

Similar to embodiment 2, the silicon substrate itself served as a commongate electrode of the five FETs and the silicon oxide film served as acommon gate insulating film of the five FETs. Thus, the FET of thestructure shown in FIG. 1 was produced. In addition, reference numerals1 and 2 designate the silicon substrate and the gate electrode,reference numeral 3 designates the silicon oxide insulating film,reference numeral 4 designates the LB film of poly (2,5-thienylenevinylene) obtained from the LB film of poly (2,5-thienylene vinylene)precursor as the semiconductor film, and reference numerals 5 and 6designate platinum source and drain electrodes, respectively.

Embodiment 4

The temperature of the same FET substrate as used in embodiment 1 andits ambient temperature was approximately 60° C. and a precursor filmwas formed on the FET substrate by spin casting using adimethylformamide (DMF) solution with approximately 2 wt % of a poly(2,5-thienylene vinylene) precursor having the following chemicalstructure ##STR14## The rotational speed of the spinner was 2000/min.The precursor film was approximately 800 Å thick. The solvent wasevaporated to some degree and the temperature of the FET substrate andits ambient was approximately 60° C. and a poly (p-phenylene vinylene)precursor film was formed on the poly (2,5-thienylene vinylene)precursor by spin casting using an aqueous solution with approximately 2wt % of the poly (p-phenylene vinylene) precursor having the followingchemical structure ##STR15## The rotational speed of the spinner was2000/min. and the thickness of the obtained precursor film was 700 Å.

The FET substrate covered with two-layer films of the poly(2,5-thienylene vinylene) precursor film and the poly (p-phenylenevinylene) precursor film was heated to 210° C. for two hours in anitrogen flow in an infrared image furnace. As a result, the color ofthe film changed from light yellow to dark brown or dark purple. Thelaminated films comprising the poly (2,5-thienylene vinylene) precursorfilm and the poly (p-phenylene vinylene) precursor film were changed tolaminated films comprising a poly (2,5-thienylene vinylene) film andpoly (p-phenylene vinylene) through the above heating treatment andaccordingly absorption in accordance with C═C of poly (2,5-thienylenevinylene) at 1590 cm¹ and absorption in accordance with C═C of poly(p-phenylene vinylene) at 970 cm⁻¹, respectively, appeared in theinfrared absorption spectrum. Meanwhile, no corrosion or the like causedby acid occurred in the element formed of poly (2,5-thienylene vinylene)during the heat treatment.

The silicon oxide film on the other surface of the FET substrate wasremoved and an alloy of gallium and indium was applied to the uncoveredsilicon surface, whereby ohmic contact was made.

The silicon substrate served as a common gate electrode of the five FETsand the silicon oxide film served as a common gate insulating film ofthe five FETs. Thus, the FET shown in FIG. 3 was produced. In addition,reference numerals 1 and 2 designate the silicon substrate and the gateelectrode, reference numeral 3 designates the silicon oxide insulatingfilm, reference numeral 4 designates the poly (2,5-thienylene vinylene)film obtained from the poly (2,5-thienylene vinylene) precursor film asthe semiconductor film, reference numerals 5 and 6 designate gold sourceand drain electrodes, respectively, and reference numeral 13 designatesthe poly (p-phenylene vinylene) film obtained from the poly (p-phenylenevinylene) precursor film as the acid giving film.

Embodiment 5

An example of a method for manufacturing a liquid crystal displayapparatus having the structure shown in FIG. 2 will be describedhereinafter. An n-type silicon substrate (25 mm×40 mm) having aresistivity of 4 to 8 Ωcm and a thickness of 300 microns was thermallyoxidized, whereby oxide films (Sio₂ films) having a thickness ofapproximately 900 Å were formed on both surfaces. Similar to embodiment1, the gold electrodes (under layer chrome 200 Å and gold 300 Å )serving as the source electrode 5, the drain electrode 6, and anelectrode 7, shown in FIG. 2, were formed on that surface. In addition,both the source electrode 5 and the drain electrode 6 had an effectivearea of 2 mm×4 mm, and they were separated by a width of 3 microns. Morespecifically, the channel width was 2 mm, and the channel length was 3microns in the FET. In addition, the electrode 7 had an effective areaof 17×19 mM². This substrate is referred to as a liquid crystal displayapparatus substrate hereinafter. Similar to embodiment 1, a poly(2,5-thienylene vinylene) precursor film was formed on the liquidcrystal apparatus substrate using a DMF solution with approximately 2 wt% of the poly (2,5-thienylene vinylene) precursor.

The poly (2,5-thienylene vinylene) precursor film, other than the FETpart of the liquid crystal display apparatus substrate, was washed usingchloroform. Thereafter, this substrate was heated to 200° C. forapproximately one hour in a nitrogen flow containing approximately 1% ofhydrogen chloride gas in an infrared image furnace. Thus, only the FETpart was covered with the poly (2,5-thienylene vinylene) film and theFET part 11 in the liquid crystal display apparatus shown in FIG. 2 wascompleted.

Orientation processing was performed by obliquely depositing SiO₂ on theliquid crystal apparatus substrate and an oppositely arranged glassplate 10 having an ITO electrode 9 formed thereon so that orientation ofthe liquid crystal material occurred. A polyester film with a thicknessof 10 microns was interposed between the liquid crystal displaysubstrate and the oppositely arranged glass plate 10 having the ITOelectrode 9, except for one part so that the liquid crystal display partis invisible. The neighborhood of the liquid crystal display part wassealed with an epoxy resin, also except for one part. A guest and hostliquid crystal material (made by Merck Company with the trade nameZLI1841) was injected through this unsealed part, and then that part wassealed with the epoxy resin. A polarizing plate was adhered to the glassplate 10 and the liquid crystal display part 12 in the liquid crystaldisplay apparatus was completed.

Finally, the SiO₂ on the back surface of the liquid crystal displayapparatus substrate was partially removed, and an alloy of gallium andindium was applied thereto, whereby an ohmic contact was made. Then, alead wire was attached thereto with silver paste so that the liquidcrystal display apparatus was completed.

Embodiment 6

Similar to embodiment 1, an LB film (100 layers) of a poly(2,5-thienylene vinylene) precursor was formed on the liquid crystaldisplay apparatus substrate using a solution mixed with 0.5 ml of theDMF solution with approximately 2 wt % of the poly (2,5-thienylenevinylene) precursor and 9.5 ml of chloroform as a developer.

Then, the LB film of the poly (2,5-thienylene vinylene) precursor otherthan the FET part of the liquid crystal display apparatus substrate waswashed using chloroform. This substrate was heated to 90° C. forapproximately one hour and a half in a nitrogen flow containingapproximately 1% of hydrogen chloride gas in an infrared image furnace.Thus, only the FET part was covered with the LB film of poly(2,5-thienylene vinylene) and the FET part 11 in the liquid crystaldisplay apparatus shown in FIG. 2 was completed.

Orientation processing was performed by obliquely depositing SiO₂ on theliquid crystal apparatus substrate and an oppositely arranged glassplate 10 having an ITO electrode 9 so that orientation of a liquidcrystal occurred. Then, a polyester film with a thickness of 10 micronswas interposed between the liquid crystal display apparatus substrateand the oppositely arranged glass plate 10 having the ITO electrode 9,except for one part so that the liquid crystal display part was visible.The neighborhood of the liquid crystal display part was sealed with anepoxy resin, also except for one part. The guest and host liquid crystalmaterial (made by Merck Company with the trade name ZLI1841) wasinjected through this unsealed part, and then that part was sealed withthe epoxy resin. A polarizing plate was adhered to the glass plate 10,and the liquid crystal display part 12 in the liquid crystal displayapparatus was completed.

Finally, the SiO₂ on the back surface of the liquid crystal displayapparatus substrate was partially removed and an alloy of gallium andindium was applied thereto, whereby an ohmic contact was made. A leadwire was attached thereto with silver paste, so that the liquid crystaldisplay apparatus was completed.

Embodiment 7

An example of a method for manufacturing a liquid crystal displayapparatus having the structure shown in FIG. 5 will be shownhereinafter. Similar to embodiment 4, a poly (2,5-thienylene vinylene)precursor film was formed on the liquid crystal apparatus substrateusing a DMF solution with approximately 2 wt % of the poly(2,5-thienylene vinylene) precursor. Then, similar to embodiment 4, apoly (p-phenylene vinylene) precursor film was formed on the poly(2,5-thienylene vinylene) precursor film using an aqueous solution withapproximately 2 wt % of the poly (p-phenylene vinylene) precursor. Thepoly (2,5-thienylene vinylene) precursor and the poly (p-phenylenevinylene) precursor film, other than the FET part of the liquid crystaldisplay apparatus substrate, was washed using chloroform. Thereafter,this substrate was heated to 200° C. for approximately one hour in anitrogen flow in an infrared image furnace. Thus, only the FET part wascovered with poly (2,5-thienylene vinylene) and poly (p-phenylenevinylene), and the FET part 11 in the liquid crystal display apparatusshown in FIG. 5 was completed. Then, the liquid crystal display part 12in the liquid crystal display apparatus was completed by the same stepsdescribed for embodiment 5. Then, similar to embodiment 5, the liquidcrystal display apparatus was completed.

Comparative Example

An element as a comparative example was manufactured in accordance withApplied Physics Letters, Volume 49, page 1210, 1986. More specifically,a reaction solution was made by dissolving 0.15 g of 2,2'-dithiophene asa monomer and 0.55 g of tetraethylammonium perchlorate as an electrolytein 75 ml of acetonitrile. This solution was aerated with nitrogen gas ofhigh impurity. The FET substrate obtained in embodiment 1 was dippedtherein. Then, electrochemical polymerization was performed by applyinga constant current (100 μA/cm²) between opposite platinum electrodes (10mm×20 mm) for 480 seconds using five pairs of gold electrodes on the FETsubstrate as working electrodes, whereby a polythiophene film with athickness of 1400 Å was formed on the five pairs of gold electrodes andthe silicon oxide film. Since a large amount of perchlorate ions weredoped into the polythiophene film at the time of the electrochemicalpolymerization, the potential of the five pairs of gold electrodes wasset at 0 V with reference to a saturated calomel electrode, and dedopingwas performed immediately after the electrochemical polymerization sothat the polythiophene had the electrical conductivity of asemiconductor layer. The FET was washed with acetonitrile two times andthen dried in a vacuum desiccator.

Next, the characteristics of the devices obtained in accordance withembodiments 1 to 7 and the comparative example will be described.

FIG. 6 shows electrical characteristics of one of five FETs inaccordance with embodiment 1. Referring to this figure, the abscissashows the voltage (V_(DS)) between the source and drain and the ordinateshows the current (I_(S)) between the source and drain. When the gatevoltage (V_(G)) is at 0 V, there is almost no I_(S) even if V_(DS) isincreased. In addition, I_(S) is saturated when V_(DS) is large so thatthe typical electrical characteristics of an enhancement type fieldeffect transistor are obtained. As can be seen from this figure, thecurrent between the source and drain has a large variation with appliedgate voltage. Although FIG. 6 shows the characteristics of one of thefive FETs, the other FETs have the same characteristics as shown in FIG.6. In addition, when their electrical characteristics are measured afterthese FETs have been left in air for approximately one month, it wasfound that the characteristics hardly changed and that the FETs producedin accordance with this embodiment have high stability.

Next, FIGS. 7 and 8 show electrical characteristics of one of five FETsproduced in accordance with embodiment 2 and electrical characteristicsof one of five FETs produced in accordance with embodiment 3,respectively. Referring to these figures, the abscissa shows the voltage(V_(DS)) between the source and drain and the ordinate shows the current(I_(S)) between the source and drain. When the gate voltage (V_(G)) isat 0 V, there is almost no I_(S) even if V_(DS) is increased. However,when a negative V_(G) is applied, I_(S) is increased. In addition, I_(S)is saturated when V_(DS) is large so that the typical electricalcharacteristics of an enhancement type field effect transistor areobtained. As can be seen from these figures, the current between thesource and drain can significantly vary with an applied gate voltage.Although FIGS. 7 and 8 each show the characteristics of one of the fiveFETs in accordance with each embodiment, the other FETs show the samecharacteristics as that shown in FIGS. 7 and 8. In addition, when theirelectrical characteristics are measured after these FETs are left in airfor approximately one month, it is found that their characteristics arehardly changed and the FETs produced in accordance with theseembodiments have high stability.

FIG. 9 shows electrical characteristics of one of five FETs produced inaccordance with embodiment 4. Referring to this figure, the abscissashows the voltage (V_(DS)) between the source and drain and the ordinateshows the current (I_(S)) between the source and drain. Similar toembodiment 1, typical electrical characteristics of an enhancement typefield effect transistor are also obtained in this case. As can be seenfrom this figure, a large variation in the current between the sourceand drain can be produced with an applied gate voltage as compared withembodiment 1 shown in FIG. 6.

FIG. 10 shows characteristics of the current flow between the source anddrain as a function of gate voltage for one of the five FETsmanufactured in accordance with each of embodiments 1 and 4 and an FETmanufactured in accordance with the comparative example with a constantvoltage between the source and drain of -50 V. Referring to this figure,the abscissa shows the gate voltage (V_(G)) and the ordinate shows thecurrent (I_(S)) between the source and drain. As can be seen from FIG.10, the current between the source and drain is modulated by the gatevoltage over four orders of magnitude in the FET manufactured inaccordance with embodiment 1. In addition, the modulatable currentbetween the source and drain is five orders of magnitude or more in theFET manufactured in accordance with embodiment 4. Meanwhile, the currentbetween the source and drain that can be modulated by the gate voltageis only two and a half orders of magnitude in the conventional FET ofthe comparative example. Thus, the characteristics of FETs in accordancewith embodiments 1 and 4 are highly improved as compared with theconventional FET.

FIG. 11 shows characteristics of the current between the source anddrain as a function of the gate voltage of one of the five FETsmanufactured in accordance with embodiment 2, one of the five FETsmanufactured in accordance with embodiment 3 and the FET manufactured inaccordance with the comparative example with a constant voltage betweenthe source and drain of 50 V. Referring to this figure, the abscissashows the gate voltage (V_(G)) and the ordinate shows the current(I_(S)) between the source and drain. As can be seen from FIG. 11, thecurrent between the source and drain is modulated by the gate voltageover four orders of magnitude or more in the FETs manufactured inaccordance with embodiments 2 and 3. Meanwhile, the current between thesource and drain is modulated by the gate voltage over only two and ahalf orders of magnitude in the conventional FET in accordance with thecomparative example. Thus, the characteristics of the FETs in accordancewith embodiments 2 and 3 are highly improved as compared with theconventional FET.

FIG. 12 shows characteristics of the current between the source anddrain and the voltage between the source and drain of the FET in theliquid crystal display apparatus manufactured in accordance withembodiment 5 when the gate voltage is varied. Referring to this figure,the abscissa shows the voltage (V_(DS)) between the source and drain andthe ordinate shows the current (I_(S)) between the source and drain. Inthis figure, when the gate voltage of the FET is at 0 V, even if avoltage is applied between the source electrode and the drain electrode,there is almost no current flow between the source and drain. However,the more negative the gate voltage is, the larger is the current thatflows between the source and drain. Since the FET is connected in seriesto the liquid crystal display part, when a negative voltage is appliedto the gate electrode 2 and a voltage sufficient to drive the liquidcrystal material 8 is applied between the transparent electrode 9 on theglass plate 10 of the liquid crystal display part and the sourceelectrode 5 of the FET, a voltage is applied to the liquid crystaldisplay part and then the liquid crystal material 8 is oriented so thatthe liquid crystal display part is driven. However, when the gatevoltage is set at 0 V, a voltage is not applied to the liquid crystaldisplay part and driving of the liquid crystal display part is stopped.More specifically, driving of the liquid crystal display is controlledby the FET in which the attached π-conjugated polymer film is thesemiconductor film. In addition, the liquid crystal display apparatusmanufactured in accordance with this embodiment still stably operatedafter more than one month.

FIG. 13 shows characteristics of the current between the source anddrain and the voltage between the source and drain when the gate voltageof the FET in the liquid crystal display apparatus manufactured inaccordance with embodiment 6 is varied. Referring to this figure, theabscissa shows the voltage (V_(DS)) between the source and drain and theordinate shows the current (I_(S)) between the source and drain. In thisfigure, when the gate voltage of the FET is 0 V, even if a voltage isapplied between the source electrode and the drain electrode, there isalmost no current between the source and drain. However, the morenegative the gate voltage is, the larger is the current that flowsbetween the source and drain. Since the FET is connected in series tothe liquid crystal display part, when a negative voltage is applied tothe gate electrode 2 and a voltage sufficient to drive the liquidcrystal 8 is applied between the transparent electrode 9 on the glassplate 10 of the liquid crystal display part and the source electrode 5of the FET, a voltage is applied to the liquid crystal display part andthen the liquid crystal material 8 is oriented so that the liquidcrystal display part is driven. However, when the gate voltage is at 0V, a voltage is not applied to the liquid crystal display part anddriving of the liquid crystal display part is stopped. Morespecifically, driving of the liquid crystal material can be controlledby the FET in which the attached LB film of π-conjugated polymer is thesemiconductor film. In addition, the liquid crystal display apparatusmanufactured in accordance with this embodiment has stably operatedafter more than one month.

FIG. 14 shows characteristics of the current between the source anddrain and the voltage between the source and drain of the FET in theliquid crystal display apparatus manufactured in accordance withembodiment 7 when the gate voltage is varied. Referring to this figure,the abscissa shows the voltage (V_(DS)) between the source and drain andthe ordinate shows the current (I_(S)) between the source and drain. Ascan be seen from the figure, the current between the source and drainwhen the gate voltage is applied is larger than the current ofembodiment 5 shown in FIG. 12, so that the characteristics are improved.In addition, similar to embodiment 5, driving of the liquid crystalmaterial is controlled by the FET. Furthermore, the stability is alsothe same as in embodiment 5.

Although the liquid crystal display apparatus is made by manufacturingone FET and one liquid crystal display part in embodiments 5 to 7, it ispossible to make the liquid crystal display apparatus by manufacturing aplurality of FETs and liquid crystal display parts. In this case,however, processing such as patterning using photoresist is necessary.

INDUSTRIAL APPLICABILITY

As described above, the present invention relates to a field effecttransistor using an organic semiconductor and a liquid crystal displayapparatus using this transistor. The invention is used in a field effecttransistor or in a liquid crystal display apparatus using the transistoras a driving element.

We claim:
 1. A field effect transistor comprising:a source electrode; adrain electrode; a semiconductor layer serving as a current path betweensaid source and drain electrodes and formed of a π-conjugated polymerwhich is obtained from a precursor which is soluble in a solvent; anacid giving film disposed in contact with the semiconductor layer andproducing an acid in a reaction in which the π-conjugated polymer isobtained from said precursor which is soluble in the solvent; aninsulating film disposed on said semiconductor layer; and a gateelectrode disposed on said insulating film opposite from saidsemiconductor layer for controlling the electrical conductivity of saidsemiconductor layer in response to a voltage applied to said gateelectrode.
 2. A field effect transistor in accordance with claim 1,wherein said π-conjugated polymer which is obtained from a precursorwhich is soluble in a solvent is represented by the following generalformula ##STR16## where R₁ and R₂ are one of --H, alkyl group, andalkoxyl group and n is an integer of at least
 10. 3. A field effecttransistor in accordance with claim 1, wherein said acid giving film isa π-conjugated polymer represented by the following general formula##STR17## where R₆ is one of --H, alkyl group, and alkoxyl group and nis an integer of at least
 10. 4. A liquid crystal display apparatuscomprising:a driving part formed of a field effect transistor having: asource electrode; a drain electrode; a semiconductor layer serving as acurrent path between said source and drain electrodes and formed of aπ-conjugated polymer which is obtained from a precursor which is solublein a solvent; an acid giving film disposed in contact with saidsemiconductor layer and producing an acid in a reaction in which saidπ-conjugated polymer is obtained from said precursor which is soluble inthe solvent; an insulating film disposed on said semiconductor layer;and a gate electrode disposed on said insulating film opposite from saidsemiconductor layer for controlling the electrical conductivity of saidsemiconductor layer in response to a voltage applied to said gateelectrode; and a liquid crystal display part connected in series to oneof said source electrode and drain electrode and controlled by applyinga voltage to said gate electrode.
 5. A field effect transistorcomprising:a source electrode; a drain electrode; a semiconductor layerserving as a current path between said source and drain electrodes andformed of a π-conjugated polymer which is obtained from a precursorwhich is soluble in a solvent, the π-conjugated polymer having thegeneral formula ##STR18## where R₁ and R₂ are one of --H, alkyl group,and alkoxyl group and n is an integer of at least 10; an insulating filmdisposed on said semiconductor layer, the insulating film producing anacid in a reaction in which the π-conjugated polymer is obtained fromsaid precursor; and a gate electrode disposed on said insulating filmopposite from said semiconductor layer for controlling the electricalconductivity of said semiconductor layer in response to a voltageapplied to said gate electrode.